@classmethod @staticmethod

Python Classes and Functions

• New Classes and old classes. Diffreence ?
• Though classmethod and staticmethod are quite similar, there's a slight difference in usage for both entities: classmethod must have a reference to a class object as the first parameter, whereas staticmethod can have no parameters at all.
• Classmethods leads to Factory Design pattern in python

Python @staticmethod

Static methods are a special case of methods. Sometimes, you'll write code that belongs to a class, but that doesn't use the object itself at all. For example:

class Pizza(object):
    @staticmethod
    def mix_ingredients(x, y):
        return x + y
 
    def cook(self):
        return self.mix_ingredients(self.cheese, self.vegetables)

In such a case, writing mix_ingredients as a non-static method would work too, but it would provide it a self argument that would not be used. Here, the decorator @staticmethod buys us several things:

Python doesn't have to instantiate a bound-method for each Pizza object we instiantiate. Bound methods are objects too, and creating them has a cost. Having a static method avoids that:

>>> Pizza().cook is Pizza().cook
False
>>> Pizza().mix_ingredients is Pizza.mix_ingredients
True
>>> Pizza().mix_ingredients is Pizza().mix_ingredients
True

It eases the readability of the code: seeing @staticmethod, we know that the method does not depend on the state of object itself; It allows us to override the mix_ingredients method in a subclass. If we used a function mix_ingredients defined at the top-level of our module, a class inheriting from Pizza wouldn't be able to change the way we mix ingredients for our pizza without overriding cook itself.

Python @classmethods

Having said that, what are class methods? Class methods are methods that are not bound to an object, but to… a class!

>>> class Pizza(object):
...     radius = 42
...     @classmethod
...     def get_radius(cls):
...         return cls.radius
... 
>>> 
>>> Pizza.get_radius
<bound method type.get_radius of <class '__main__.Pizza'>>
>>> Pizza().get_radius
<bound method type.get_radius of <class '__main__.Pizza'>>
>>> Pizza.get_radius is Pizza().get_radius
True
>>> Pizza.get_radius()
42

Whatever the way you use to access this method, it will be always bound to the class it is attached too, and its first argument will be the class itself (remember that classes are objects too). When to use this kind of methods? Well class methods are mostly useful for two types of methods: Factory methods, that are used to create an instance for a class using for example some sort of pre-processing. If we use a @staticmethod instead, we would have to hardcode the Pizza class name in our function, making any class inheriting from Pizza unable to use our factory for its own use.

class Pizza(object):
    def __init__(self, ingredients):
        self.ingredients = ingredients
 
    @classmethod
    def from_fridge(cls, fridge):
        return cls(fridge.get_cheese() + fridge.get_vegetables())

Static methods calling static methods: if you split a static methods in several static methods, you shouldn't hard-code the class name but use class methods. Using this way to declare ou method, the Pizza name is never directly referenced and inheritance and method overriding will work flawlessly

class Pizza(object):
    def __init__(self, radius, height):
        self.radius = radius
        self.height = height
 
    @staticmethod
    def compute_area(radius):
         return math.pi * (radius ** 2)
 
    @classmethod
    def compute_volume(cls, height, radius):
         return height * cls.compute_area(radius)
 
    def get_volume(self):
        return self.compute_volume(self.height, self.radius)

Efficient Use Cases with abstract, class and static methods